Tendon injuries constitute an increasing proportion of trauma cases reported at our hospital, yet research in tendon healing has been neglected. There is a strong clinical impression the exercise speeds healing in an injured tendon. The hypothesis invoked to explain this phenomenon is that early motion of the affected tendon favorably influences collagen synthesis and maturation, particularly the quantity of and types of collagen crosslinks formed. The variety and amount of collagen crosslinks present are directly related to the strength and mobility of the healing tendon. Without the appropriate types of crosslinks in the appropriate locations within collagen molecules, connective tissue would literally fall apart. On the other hand, too many crosslinks in joint tissue or tendon can restrict mobility. Since the tensile strength of tendons is dependent on the nature of the collagen, and since physical activity strengthens tendons, exercise must favorably alter the crosslink chemistry of healing tendons. Moreover, a study of how cultures of tendon cells react, biochemically, to stress in vitro will indicate how stress in vivo acts to strength healing tendons.